This invention relates generally to analog based scrambling decoding techniques and more particularly relates to the method and apparatus for unscrambling a frequency-hopping analog frequency inversion scrambled signal without advance information about the sequence of frequencies used for inversion.
A relatively simple and inexpensive technique for providing secure communications over a narrow bandwidth channel utilizes frequency inversion of the input analog signal to introduce security. An input analog signal is conventionally mixed with a single frequency tone in a square-law mixer or balanced modulator and the lower sideband of the product of the single frequency tone and the input analog signal is selected by a filter. The resultant output signal is one in which the analog signal has the lowest frequency components and highest frequency components reversed and shifted in frequency.
A more secure technique changes the frequency of the mixing tone at predetermined time intervals such that more than one tone is used over a period of time to invert the input analog signal. In this way, the inversion frequency hops among several tone frequencies according to a predetermined sequence of tone frequencies and provides additional security for the scrambled signal. Such a system is further described in U.S. Pat. No. 4,827,507. The apparatus and method outlined in this patent requires that the predetermined sequence of tones used in the inversion process be communicated between the transmitter transmitting the scrambled signal and the receiver receiving the scrambled signal so that the scrambled signal can be unscrambled. However, under some circumstances it is desirable to unscramble the scrambled transmission without the inversion tone hopping sequence being communicated.